Agricultural tractors provide a variety of functions on a modern farm, including cultivating fields with a variety of implements, towing implements and trailers, and excavating with front end loaders. The implements are typically coupled to the tractor using several mechanical linkages, which permit the implement to be raised and lowered with respect to the tractor by hydraulic cylinders or other actuators. A common hitch provided on many agricultural tractors is the "three-point hitch." This hitch permits an implement to be coupled to the tractor and supported as an overhanging load, or lowered and forced into the ground. Plows and other ground cultivating implements are typically supported in this manner.
During typical farm operations, tractors with their attached implements in a raised position travel over roads at relatively high speeds to go from field to field. During these transport phases, with the implement raised the suspension, or the lack thereof, causes the tractor to bounce and pitch as it travels. Depending upon the weight of the implement, and the corresponding degree of bouncing and pitching, it can be difficult to control the tractor.
In an attempt to improve roadability, various systems have been developed for interacting with the implements, their associated mechanical linkages and hydraulics to control bouncing and pitching while operating at road speeds. One such system includes circuitry for lifting and tilting an implement combined with a shock absorbing mechanism. This system permits relative movement between the implement and the vehicle to reduce pitching of the vehicle during road travel. Other systems for improving the performance of vehicles have included accumulators, which are connected and disconnected to the hydraulic system depending upon the speed of the vehicle.
Some systems use existing instrumentation on the vehicle, such as load sensors adapted to measure the load on the hitch and position sensors adapted to measure the height of the hitch, to actively control the position of the implement with respect to the vehicle while the vehicle is moving down the road. For example, U.S. Pat. No. 4,924,943 (issued to Maichle, May 15, 1990) discloses an Automatic Pitch Vibration Damping Mechanism for an agricultural tractor that monitors the hitch position and load using position and load sensors that are employed when the implement is being pulled through the earth to control the load and the depth of the implement. In the Maichle reference, the control system provides a signal to the control valve that lifts or lowers the implement. This valve signal is the sum of two signals, a position signal that is a function of the difference between the actual position of the hitch and a nominal hitch position, and a force signal that is a function of the difference between the actual force on the hitch and a nominal force. The force regulating circuit 36 provides a signal to the control valve related to the force on a load pin, and the position regulating circuit 42 provides a signal to the control valve that is related to the distance the hitch is from a nominal position. These two signal components are summed at block 39 and applied to the control valve to move the hitch.
The Maichle reference describes several limitations to his system. The force signal is based on a nominal force created by low-pass filtering the actual force. This nominal force is perturbed when the vehicle goes around a curve causing the implement to rise until it hits a mechanical stop or abutment in the curve, and causing the implement to fall until it hits the pavement once out of the curve. The curve-hitting problem is limited by preventing the nominal force value from changing rapidly. Whenever the system raises the implement up high (e.g. when the control system overcorrects upwardly in the curve) the nominal force value is permitted to change quite slowly by providing a large time constant. These problems arise from the use of a reference hitch force created by taking the time average (i.e. low pass filtering) of the actual hitch force.
What is needed is an improved system for reducing the pitching of a tractor that positions the tractor hitch without requiring a hitch position-based variable low-pass filter time constant for calculating a nominal force value that is combined with an actual force value and presented to a regulator to be later combined with a position error calculation and applied to a valve constant.